Electric machines, in particular electric motors, can be very well controlled with respect to their rotational speed. One known means of improving this basic controllability is to provide mechanical brakes, which can be released or raised by means of an electromagnetic apparatus. While the electric motor is in operation, the current powering the electric motor is also supplied to an excitation coil of the electromagnetically actuated mechanical brake. When no current is thus supplied, the electric motor is immobilized by application of a spring force to the brake.
Furthermore, a known means of powering electric motors is by a mains supply of alternating or polyphasic current. In this case, if the motor incorporates an excitation coil operated by direct current, converters or rectifiers are employed. Because the operating voltages of a.c. or polyphasic mains vary in different parts of the world, this equipment is manufactured and marketed in a number of variants, with excitation coils for different operating voltages. For the same reason, diverse rectifier circuit arrangements are produced. In order to reduce costs by increasing the production output, efforts are already being made to introduce a rectifier circuit arrangement that can be operated in several voltage categories that are used globally. However, because the amplitude of the unipolar voltage generated by the circuit arrangement depends on the a.c. voltage amplitude, the excitation coil of the electromagnetically actuated mechanical brake must be selected to be entirely compatible with the mains voltage available at the site where the brake is to be used.
The object of the invention is to develop a method and a circuit arrangement for the operation of an electromagnetically actuated mechanical brake of an electric motor, in particular a geared motor, in such a way that the same type of excitation coil can be used with a.c. or three-phase mains supplies of different voltages.